Brightening clouds: Atmospheric scientists evaluate a technique for reflecting more sunlight back to space

August 11, 2011
When aerosol particles from ship exhaust enter the lower atmosphere, marine stratocumulus clouds are brightened, leaving “ship tracks” visible in satellite images. Credit: Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC.

( -- What happens when tiny seawater particles are intentionally injected into low clouds over the ocean? To answer this question, scientists at Pacific Northwest National Laboratory and the National Oceanic and Atmospheric Administration developed a high-resolution model to better understand the effects of particle injection and evaluate whether this technique could be used to offset some effects of global climate change. The study was published in the journal Atmospheric Chemistry and Physics.

Droughts. Floods. . To alleviate some of the consequences of , some climate experts propose geo-engineering, the deliberate manipulation of the planet's climate. One such proposal calls for offsetting by brightening marine stratocumulus clouds. Brighter clouds reflect more sunlight back into space, reducing the amount of heat absorbed by the earth. While not an endorsement of this proposal, the PNNL-NOAA research provides fundamental information for evaluating the practicality of cloud brightening and similar geo-engineering concepts.

"The geo-engineering concept of cooling the Earth by making clouds brighter was proposed more than two decades ago, but we are still at a very preliminary exploration stage. The purpose of our research is to not only better understand the physics of cloud brightening, but also understand potential unintended consequences, such as shifting or effects on " said Dr. Hailong Wang, a PNNL who led the research team.

Located in the boundary layer, the lowest level of the earth's atmosphere, marine stratocumulus clouds cover vast areas of the ocean surface. When from ship exhaust enter these clouds, tell-tale "ship tracks" are visible in satellite images. One proposal to offset some effects of climate change envisions a global fleet of wind-driven, unmanned ships designed to spray tiny seawater particles into low ocean clouds. Like ship exhaust, injected aerosols composed of seawater particles brighten clouds by causing water droplets become smaller and more numerous.

The PNNL-NOAA research team ran model simulations of aerosol injection using a high-resolution version of the Weather Research and Forecasting (WRF) model to investigate how the injected particles are transported through the marine boundary layer and how they affect the microphysical processes inside clouds. Interactions of aerosols, clouds and rain were studied across several sets of experiments. Each set combined different meteorological and aerosol background conditions observed off the coast of California. The injection of seawater particles was simulated in the model using moving point sources, closely mimicking the behavior of the conceptual seawater spraying vessels.

The simulations confirm that aerosol injection does brighten clouds, but the amount of solar radiation reflected may not be enough to balance the global warming caused by burning fossil fuels. The amount of sunlight reflected also depends on several factors, including the distribution of injected aerosols, ambient levels of water vapor and pollution, and the timing of aerosol injections (because cloud brightening is only meaningful in the daytime). Also, the PNNL-NOAA study team found that adding aerosols changes the amount of rain falling from , which is an important result because changes in rainfall and other climate variables may have even larger impacts than changes in temperature alone.

The research team will compare the model results with satellite and in situ observations of ship tracks and other disturbances and will extend the computer model experiments to other geographical regions, meteorological conditions, and cloud types.

Explore further: Geoengineering takes a ride in the shipping lanes

More information: Wang H, et al: "Manipulating marine stratocumulus cloud amount and albedo: a process-modelling study of aerosol-cloud-precipitation interactions in response to injection of cloud condensation nuclei," Atmospheric Chemistry and Physics, 11, 4237-4249, DOI:10.5194/acp-11-4237-2011 , 2011.

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3.7 / 5 (3) Aug 11, 2011
I'm reminded of a quote from the Matrix:

"We don't know who struck first, us or them. But we do know it was us that scorched the sky."

I am against any attempts to intentionally tinker with our biosphere. We know precious little about what our current existence has on it, and our history is rife with examples of environmental and ecological experiments gone wrong.
1 / 5 (1) Aug 11, 2011
and ones gone right, like fixing the ozone layer....

Do nothing is always the wrong attitude; but hey whatever, this country is so anti science now. I'm just waiting for Mr. Manuel to show up and preach the scientific conspiracy theory again.
2.6 / 5 (5) Aug 11, 2011
"Fixing" the ozone layer was done by ban treaties and time, not by geo-engineering.
not rated yet Aug 11, 2011
"Fixing" the ozone layer was done by ban treaties and time, not by geo-engineering.

Exactly. No pie in the sky cloud-seeding or other nonsense.

Stopping a harmful change that we are having is much, much different than trying a new change that we hope might offset the other bad effects we are causing.
3 / 5 (2) Aug 12, 2011
The problem with all this geo-engineering is who WILL control is when its all in place? Will we expect weather based terror attacks soon?
5 / 5 (2) Aug 12, 2011
Geoengineering (like seeding the oceans with iron, brightening the clouds or whatnot) is way too moncausalistic.

It's nearsighted to say: "here is an effect we want to produce and we have a tool that can produce it" without taking into account that global system are very complex and tightly coupled.

Any engineer can tell you that by pushing on a complex system in one place you never only get an effect in another. You always get a slew of side effects.

On a smaller scale most can comprehend this: There is no side-effect free medication. Only those with side effects that we can tolerate. Drugs, however, are tested within a limited scope first and 9 out of 10 drugs don't see the market because of unforeseen side effects.

How would we test out this scheme? We don't have a limited bio/geosphere all set up. Simulations aren't enough before trying something on this scale. If it goes wrong we'll be royally screwed.
1 / 5 (2) Aug 16, 2011
Simulations aren't enough before trying something on this scale. If it goes wrong we'll be royally screwed.

On this I agree 100%. Let me take that a bit further, though. Simulations of any kind are insufficient to the task of making major decisions of any kind. They are, in effect, climate models but they are simulations nonetheless, as are...well...climate models. :)

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